1. Field of the Invention
The present invention relates generally to micropositioning, and particularly to glass-based micropositioning systems and methods.
2. Technical Background
A micropositioner is a device capable of placing a small object in a desired location and holding it there with a relatively high degree of accuracy and precision. Such positioning is sometimes referred to as either “micropositioning” or “microalignment.” Micropositioners (also referred to as “microactuators”) come in many different forms, such as electromagnetic, mechanical, piezo-electric, micro-electro-mechanical (MEMS), etc., and are used for a wide variety of applications.
Micropositioners are becoming increasingly important in the field of optics, particularly with respect to guided-wave and integrated optics. Planar waveguide technology, for example, enables high-density integration of optical functions, and allows for low-cost fabrication of integrated optical devices using standard semiconductor processing techniques. Hundreds of complex passive or active devices (e.g., arrayed waveguide gratings (AWGs) multiplexer/demultiplexers or semiconductor optical amplifiers (SOAs)) can be fabricated on a single planar substrate using high-index waveguide technologies.
Planar integrated optical devices generally require multiple waveguide interconnections to external optical components, which are typically provided via optical fiber pigtail interconnections. In cases where the planar waveguide mode field diameter is closely matched to that of a single-mode optical fiber (e.g., ˜9 μm), multiple external interconnections can be provided using optical fiber arrays mounted in precision fiber array pigtail blocks. These blocks, which are fabricated using silicon V-groove arrays or machined glass or ceramic blocks, typically align (low-eccentricity) fiber cores with lateral misalignments of <0.5 μm offset from ideal on-pitch positions across the fiber array.
Low-loss coupling to high-index planar waveguide arrays can be more difficult using fiber array pigtail blocks, since the smaller mode field diameters (typically 3.5-4.0 μm) are more sensitive to lateral misalignments. For example, a 0.5 μm core misalignment contributes 0.3-0.4 dB to the interconnection insertion loss. Use of fiber array pigtail blocks also requires that planar waveguides be positioned on a pitch equal to or greater than the optical fiber diameter (typically 127 μm for 125 μm diameter fiber). In some cases (e.g., SOA arrays), this pitch requirement can lead to oversized planar device chips and wasted real estate, increasing device cost. Narrow pitch SOA array waveguides are also desirable for minimizing vertical misalignments of waveguide centers due to SOA array chip warpage induced before or after chip mounting (e.g., waveguide “smile”). Use of pigtail blocks typically involves direct mechanical attachment of the fiber array block to the planar waveguide substrate using organic adhesives.
Some planar device substrates (e.g., SOA arrays) are too thin and/or small to make this attachment approach practical. Control of adhesive flow before thermal or UV curing is also an issue when making attachments to air-clad waveguides and active devices, since sudden cladding index changes can lead to unwanted back reflections and/or scattering.
In light of the difficulties mentioned above for fiber pigtail blocks, low-loss connections to small mode field diameter waveguides are often made by active micropositioning of individual optical fibers to planar waveguides. Optical fibers can be fabricated with lensed or wedged tips that provide a low-loss mode field transformation between the small planar waveguide mode and a larger single-mode fiber (SMF) waveguide mode. These optical fibers are generally actively aligned and then fixed in place using solder, laser welding or organic adhesives. In general, post-attachment lateral shifts induced by the adhesive material necessitate additional bending, nudging or (in the case of laser welding) laser hammering to bring the fiber back into alignment with the planar waveguide.
Since each planar waveguide interconnection requires a separate optical fiber alignment step, the alignment process grows increasingly complex as the number of waveguide interconnections increases. This has a direct impact on the yield of the assembly and hence, the cost. Additionally, the fixturing hardware required to hold the fiber in place during attachment (e.g., laser weld clips) generally increases the planar waveguide pitch, further increasing planar device size and cost. Laser weld attachment methods that require rebending are generally undesirable in planar array interconnection applications, since rebending the optical array interface in the plane of the planar substrate introduces unwanted axial separations between waveguides at one end of the array or the other.
Planar waveguide device micropositioning is typically provided by an external precision micropositioning system (e.g., multi-axis translation/rotation stages driven by piezo-electric micropositioners). While such systems can generally align waveguide arrays on two planar waveguide devices to within less than 0.2 μm of lateral misalignment, a critical issue is maintaining waveguide alignment during adhesive curing without post-attachment rework. Crystal block attachment is one solution for zero-shift attachment of devices to substrates, but the cantilever nature of the planar device attachment to the common alignment substrate via an intermediate block makes the approach less suitable for planar devices requiring wirebonded electrical interconnections. Additionally, a six-axis positioning system is required to align the two substrates.
An alternative planar device micropositioning and attachment approach involves integrating or mounting the micropositioners on a common alignment substrate. Such micropositioners have been based on a number of different technologies, such as expansion and/or contraction of piezo-electric materials, electro-strictive materials, magneto-strictive and magnetic materials. Such micropositioners have also been fabricated based on electrostatic forces between plates and substrates, electrically-induced shape changes in polymers, and ultrasonic excitation of flexture elements. Another common integrated micropositioner approach takes advantage of material expansion and/or contraction via thermo-mechanical effects.
All of the micropositioning techniques mentioned above generally require continuous control input (e.g., an electrical signal) in order to maintain their position. This is a significant disadvantage in micropositioning hybrid optical components because control input must be maintained over the life of the product. Further, many of the micropositioning technologies described above require complex and expensive precision microstructures (e.g., MEMS) that increase product cost.